The present invention generally relates to so-called surface-acoustic-waver devices having a plurality of electrodes, and in particular to the arrangement of input and output electrodes of such a surface-acoustic-wave device for optimizing the characteristics of the device.
Recently, the demand for increased operational speed of information processing apparatuses and communication apparatuses has caused the shift of frequency used for the carriers or signals to higher frequency regions. In correspondence to such a shift of the frequency band, filters capable of operating in such high frequency region are required. For this purpose, the surface-acoustic-wave (abbreviated hereinafter as SAW) devices such as SAW filters are used.
In view of expected developments in the future, particularly in the field of automobile telephones and portable telephones, efforts ar made to develop the SAW device having a sharp attenuation in the frequency region outside the pass-band while maintaining a uniform band-pass characteristic in the pass-band. By using the SAW device in place of the conventional dielectric filters, the size of the filter can be reduced to about 1/30 and the size of the telephone can be reduced accordingly.
A typical SAW device such as the SAW filter uses a piezoelectric substrate having large electromechanical coupling coefficients and small temperature coefficient of frequency. For example, a single crystal of LiTaO.sub.3 is used widely. The crystal of LiTaO.sub.3 is cut in a predetermined orientation, and interdigital electrodes are provided on the substrate as the input and output electrodes.
FIG. 1 shows the geometrical parameters characterizing a typical interdigital electrode.
Referring to FIG. 1, the electrode comprises a first part EL1 and a second part EL2 each having a number of fingers f.sub.1 -f.sub.n or g.sub.1 -g.sub.n, wherein each finger has a width W and separated from adjacent fingers by a separation S. Designating the wavelength of the surface acoustic wave as .lambda., the width W and the separation S are generally set to satisfy the relation W=S=.lambda./4. Thereby, the pitch defined in FIG. 1 as P is set to P=.lambda./2. Further, the each finger in the electrode EL1 and each finger in the electrode EL2 are provided to form a uniform overlap as shown in FIG. 1. Such an electrode is called the uniform overlap electrode.
When forming a SAW filter having a central band pass frequency of 835 MHz, for example, the pitch P is set to 2.45 .mu.m while the width W and the separation S are set to 1.23 .mu.m in correspondence to the velocity of 4090 m/sec of the surface acoustic wave in the X-direction. It should be noted that the foregoing velocity provides the wavelength .lambda. of 4.9 .mu.m for the surface acoustic wave of 835 MHz. Generally, a pair of such electrodes EL1 and EL2 are provided. In the particular applications of SAW devices such as automobile telephones or portable telephones, on the other hand, devices having a small insertion loss, a wide pass-band and a large suppression for the frequency components outside the pass band, are required. For example, an insertion loss of 3-5 dB or less, a pass band of 25 MHz or more and the side lobe suppression of 24-25 dB or more may be required for the SAW filter having the central frequency of 835 MHz.
In order to satisfy these various requirements, various proposals have been made, including the SAW device having the multiple electrode construction (Lewis, M., Ultrasonics Symposium Proceedings, p. 12, 1982).
FIGS. 2(A) and 2(B) show the block diagram of the multiple electrode SAW device, wherein FIG. 2(A) shows the case where an odd number of electrodes are provided and FIG. 2(B) shows the case where an even number of electrodes are provided.
In the drawings, the SAW device has a number of interdigital input electrodes 2 and a number of interdigital output electrodes 3 provided alternately on the same surface of the piezoelectric crystal (not shown). At both sides of the electrodes 2 and 3, a pair of reflectors 4 are provided, and the electrode 2 is connected to an input terminal 20, the electrode 3 to an output terminal 30.
In FIGS. 2(A) and 2(B), the number of pairs of finger electrodes that form the interdigital electrode is shown by oN for the output electrodes 3 and by iN for the input electrodes 2. Here, each pair of the finger electrodes includes one finger electrode such as the finger f.sub.2 for the electrode EL2 and an adjacent, opposing finger electrode such as the finger g.sub.1 for the opposing electrode EL1 shown in FIG. 1. FIG. 2(A) shows the SAW device having six input electrodes and five output electrodes while FIG. 2(B) shows the device having seven input electrodes and six output electrodes. In any of these, the input electrode 2 and the output electrode 3 are disposed alternately.
FIG. 3 shows the interdigital electrode forming the electrodes 2 and 3 of a conventional SAW device. As can be seen, this device has the six input electrodes 2 and five output electrodes 3.
Referring to FIG. 3, the device has a piezoelectric substrate 1, and the interdigital input and output electrodes 2 and 3 are provided on the upper major surface of the substrate 1 with a mutual separation d, where d represents the distance from the center of an input electrode 2 to the center of an adjacent output electrode 3. The reflector 4 has a so-called short circuit strip type wherein a number of electrode strips are shorted with each other at both ends thereof. For the simplicity, FIG. 3 shows the device that uses the uniformly overlapped electrode for the electrodes 2 and 3. The concept of "overlap" is defined in FIG. 1. It should be noted that FIG. 3 is a schematical drawing and the number of electrode fingers in each electrode or the number of electrode strips in the reflector is not depicted accurately.
In order to improve the characteristics of the SAW device of FIG. 3, various improvements and modifications are proposed. For example, the French patent 6911765 describes the so-called apodized electrodes wherein the overlap of the finger electrodes is changed in the input electrode 2 and the output electrode 3. On the other hand, the Japanese Laid-open patent application No. 50-40259 describes a decimated electrode construction wherein the finger electrodes in the interdigital electrode are given a weight distribution by selective removal of the finger electrode. Alternatively, there is a proposal in the Japanese Laid-open patent application No. 49-66051 in which the number of pairs of the electrode finger is changed.
FIG. 4 shows the band pass characteristic of such a multiple electrode SAW filter, wherein the vertical axis represents the attenuation or insertion loss and the horizontal axis represents the frequency. The SAW filter of this example is constructed on a 36.degree.Y-X LiTaO.sub.3 substrate and has seven input electrodes 2 and six output electrodes 3 both having the uniform overlap of the finger electrodes. The parameter iN, representing the number of opposing finger electrode pairs in the input electrode 2, is set at 19 (iN=19), while the number oN, representing the number of opposing finger electrode pairs in the output electrode 3 is set to 30 (oN=30). It should be noted that the concept of "the opposing electrode pair" used herein are defined in the schematic illustration of FIG. 1. On the other hand, the reflector 4 is constructed from 30 pairs of electrode strips shorted at both sides thereof.
In the characteristic of FIG. 4, it can be seen that such a SAW filter has an extensive side lobe adjacent to the pass-band, and because of this, the out-of-band attenuation that can be reached by the present construction is limited to only 13 dB. Further, within the pass-band, one can see a large ripple or dip in the characteristic that should be as flat as possible in this region. In addition to the foregoing problems, the SAW filter of the conventional construction has a problem in that it cannot provide a sufficiently large pass-band that is desired for the applications such as automobile telephones, portable telephones and the like. It should be noted that, conventionally, dielectric filters have been used for this purpose. However, the dielectric filter, having has a large size, has caused a problem in reducing the size of the apparatuses.